The present invention relates generally to spacecraft control systems and methods, and more particularly, to automatic apparatus and method for switching actuators used on a spacecraft.
Pervious automatic actuator switching techniques have used a zero speed tachometer loop. Such loops are disclosed in U.S. Pat. No. 5,201,833 issued to Goodzeit et al. entitled "Attitude control system with reaction wheel friction compensation", and U.S. Pat. No. 5,020,745 issued to Stetson, Jr. entitled "Reaction wheel fricton compensation using dither", for example.
U.S. Pat. No. 5,201,833 discloses a spacecraft attitude control system that uses one or more momentum or reaction wheels. Wheel bearing viscous (velocity-dependent) friction reduces the actual torque imparted to the spacecraft in response to a torque command signal. Friction compensation is achieved by applying the torque command signal to a model of an ideal, friction-free wheel, and calculating the speed which the ideal wheel achieves in response to the torque command. An error signal is generated from the difference between the ideal wheel speed and the actual wheel speed. The error signal is summed with the torque command signal to produce the wheel drive signal. This results in a closed-loop feedback system in which the actual wheel speed tends toward the ideal wheel speed, thereby causing a torque on the spacecraft which is substantially equal to that commanded.
U.S. Pat. No. 5,020,745 discloses a reaction-wheel stabilized spacecraft wherein attitude errors at wheel reversals are reduced by application of a dither component to the wheel torque command signal. This invention is used on a reaction wheel stabilized spacecraft includes a body and at least one reaction wheel mounted for rotation about an axis of the body. A controllable torquer applies torque to the wheel in response to a torque command signal to accelerate or decelerate the reaction wheel and transfer momentum to the spacecraft body. Sensors are used to sense the spacecraft attitude and generate a signal in response to the deviation of the body from a desired rotational position about the axis. A controller includes an integrator coupled to the sensor for generating an attitude-sensitive component of the torque commend signal. A summer receives the attitude-sensitive component of the torque command signal and an alternating signal and generates the dithered torque command signal.
However, using such conventional zero speed tachometer loops, operator intervention is required. Furthermore, if an actuator degrades gradually, loss of lock may occur when using conventional zero speed tachometer loops.
It would therefore be advantageous to have an improved apparatus and method for automatically switching actuators used on a spacecraft when they experience gradual degradation. It would also be advantageous to have an automatic actuator switching apparatus and method that automatically switches actuators without impacting pointing performance of the spacecraft.